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WSDOT Continues Commitment to Environmental Stewardship with Pollinator-Friendly Vegetation Management Practices

May 2015


Washington State Department of Transportation (WSDOT) has a long history of managing its roadsides for cost efficiency and environmental stewardship. Based on a detailed programmatic environmental impact study they conducted in 1993, WSDOT determined that an integrated vegetation management (IVM) program would be an effective, natural, and self-sustaining approach to maintaining Washington roadways. WSDOT IVM methods include biological control, selective herbicide use, trimming, soil improvements, native plantings, and mowing. Employing IVM is a way to consider different treatments, based on site-specific conditions, and manage long-term for safety, cost-effectiveness, and improved habitat for native plants and animals, including pollinators. To implement the program, WSDOT maintains annually updated IVM plans for all 7,000 miles of State and Federal highway corridor in the State.

Recently, WSDOT has re-evaluated the environmental and economic effects of some of its IVM practices, especially non-safety related mowing in areas with wide rights of way. The agency is targeting a 30% reduction in non-safety related mowing beginning in 2015. Their studies show that reduction in this type of mowing not only saves costs but benefits environmental conditions. In their studies, for example, they found that reducing non-safety related mowing by one-third will eliminate 23 metric tons of carbon dioxide (CO2) emissions per year. There is a corresponding increase in un-mowed plant communities’ ability to sequester CO2. By minimizing the amount of mowing that occurs for non-safety reasons, WSDOT believes it can save money, improve pollinator habitat, and reduce CO2 emissions. Based on this information, they recently adopted new mowing policies. WSDOT’s research on ways to augment existing IVM practices is important, because they gathered baseline conditions before implementing a new policy, so they will have comparative data in the coming years to measure the success of changing their practices. They also have legacy data from baseline conditions evaluated in 1993 to document successes they have already seen in roadside habitat improvement using IVM strategies.

WSDOT tailors its IVM practices as necessary to meet State and Federal requirements for protected species when working in special habitat areas. For example, although prairie habitat across Washington is becoming increasingly rare, WSDOT manages many miles of State highway through prairie landscapes, and prairie habitat supports several animals and plants protected by State and Federal laws. Prairie habitat is also very important for pollinators. WSDOT also considers special status species in construction projects such as the new alignment of US12 near Walla Walla, where ground-nesting alkali bees exist in and near the proposed right-of-way (ROW). These bees pollinate alfalfa fields, and they are very important to the agricultural community. For this project, WSDOT utilized an agreement with Washington State University to conduct a study of bee movement patterns and they evaluated alternative routes to minimize bee impacts. Impacts were unavoidable because of the large area used by farming and the number of bee beds. Because alkali bee beds must be moved every few years to minimize bee pests, WSDOT is mitigating project impacts by funding farmers to move bees to new locations to minimize harm to the alkali bee, or they may compensate farmers who decide to use leaf cutter bees instead of moving bee beds.

Methods for Success

In June 2014, WSDOT published the Roadside Policy Manual, which provides practical roadside restoration policies and guidance to minimize lifecycle costs while providing operational and environmental functions. Part of WSDOT’s new roadside policy includes reducing the area they mow along the roadside to the minimum area necessary for safety, which they hope will result in more natural stands of grass and forbs to increase forage habitat for pollinators. In some areas, such as where they retain large densely forested areas of native trees along the ROW, their mowing zone may only be a few feet from pavement up to the tree or shrub line, much less than the traditional 30-foot clear zone. WSDOT’s new policy calls for reduced mowing across most parts of the State, not just forested areas. WSDOT also has a policy of mowing after the blooming and nesting season, to ensure native flowering plants have the longest possible growing season and can produce seeds for future wildflower propagation and to allow for bird habitat. This is especially important in prairie habitat where WSDOT uses the best available science to determine the right time and the right methods to mow or whether to use other IVM practices to remove encroaching woody vegetation and invasive species like scotch broom. WSDOT will monitor the effectiveness of their new reduced mowing policies as they implement them to determine cost savings and environmental outcomes.

Outside of routine annual mowing along the road edges for safety reasons, WSDOT manages vegetation based on principles of plant succession in natural ecosystems with a goal of achieving “climax vegetation,” characterized by a stable, self-sustaining plant community. WSDOT uses herbicides cautiously, targeting non-native plants that threaten natural habitats. Hand trimming is also useful to selectively remove undesirable plants for overall habitat health. Other WSDOT vegetation design and construction policy measures include amending the soil with compost and selecting long-lived plant materials to exclude weeds and to reduce herbicide requirements.

WSDOT has always been committed to planting native flowering and fruiting species along roadways. This includes a variety of native wildflower species, which improve roadside esthetics and create healthy habitat for pollinators. Native flowering plants include primarily forbs, but may also include native woody vegetation, such as the red flowering currant. WSDOT’s roadside seed mixes include native wildflowers such as clover, poppy, and lupine in western Washington and yarrow and lupine in eastern Washington.


WSDOT faces the ongoing challenge of removing invasive plants in the ROW. Weeds such as scotch broom, star thistle, and blackberry out-compete native plants, taking over areas where they become established. Blackberry is especially difficult to control, and pollinators are attracted to it because it has such a long bloom season. WSDOT is very aware of the problem of pollinator decline and especially honeybee colony collapse, but the agency must comply with State laws requiring control of designated weed species throughout the State. The result of WSDOT’s reduced mowing program will be more non-regulated weed species going to flower and seed. While this will benefit some pollinators, it will result in more spread of “nuisance” weeds and the roadsides will look more unkempt.


WSDOT has been actively implementing and promoting IVM for decades, and they are now beginning to augment their IVM by adding new environmentally friendly practices. Some of the agency’s best practices precede the recent policy changes. In a four year period between 2003 and 2007, for example, the agency reduced herbicide use by 70 percent by using IVM methods and increasing the amount of desirable vegetative cover on road shoulders.


Based on the success of its improved vegetation management practices, WSDOT has started to develop plans for improved data collection, knowledge-sharing, and public outreach. WSDOT would like to better understand the implications of its practices on pollinators, so the agency is considering a research project funded through the National Cooperative Highway Research Program (NCHRP). In order to share best practices with other interested agencies and organizations, WSDOT has expressed interest in working with the American Association of State Highway and Transportation Officials (AASHTO) to develop a webinar on pollinator issues. WSDOT is also actively working with a team from the U.S. Forest Service on pollinator issues. They have also begun creating content for a website about pollinators and their habitats. The purpose of the website will be to inform and educate the public on what WSDOT is doing to protect pollinators and to promote pollinator health.

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